State-of-the-art active infrared (“IR”) systems require photodiode detectors that can be operated at high gain and low noise in the near infrared (“NIR”), short-wave infrared (“SWIR”), mid-wave infrared (“MWIR”) and long-wave infrared (“LWIR”) bands. Photodiodes are normally operated at low reverse bias voltages that are well below avalanche breakdown. When higher reverse bias voltages are applied to a photodiode, the photodiode operates as an avalanche photodiode (“APD”) and an internal current gain is achieved, thereby increasing the signal response or responsivity of the photodiode detector. The internal current gain that occurs under avalanche operation conditions in an APD results from the well known phenomenon of impact ionization. In impact ionization, photogenerated minority carriers from a photon absorption region of the APD, which are representative of absorbed optical energy signals, are injected into and then multiply within a carrier multiplication region of the APD.
APDs, which can be fabricated from semiconductor materials such as Si, InGaAs/InP, InP, InAlAs, etc., are designed to maximize the injection of the signal current, or photogenerated minority carriers, from the absorption region into the carrier multiplication region of the photodiode. In addition, APDs are designed to minimize noise. The noise can include excess noise, which can be a function of the gain of the APD, and noise based on dark current. Sources of dark current, as well known in the art, can include thermally generated dark currents and tunnel dark currents. By maximizing signal current and minimizing noise, high signal-to-noise level performance can be achieved in the APD, which improves the ultimate sensitivity of a detector formed from the APD.
Research on APDs has focused on obtaining higher gains while minimizing noise so that a greater responsivity can be achieved. For example, specialized architectures, such as the cylindrical architecture shown in Beck, J. D. et al., Proc. SPIE, Vol. 4454, pp. 188-197 (2001) (“Beck 2001”), incorporated by reference herein, which are distinct from those used in photodiodes normally operated in a non-APD mode and favor signal carrier injection into the carrier multiplication region over tunnel dark currents, have been developed. In addition, some APDs, such as narrow bandgap APDs fabricated from InGaAs/InP, include separate photon absorption and carrier multiplication regions that function to minimize tunneling of dark current from the narrower bandgap InGaAs absorption region into the carrier multiplication region.
In early generation APDs, excess noise of the APD, or noise in excess of the noise multiplied by the gain, substantially impacted the signal-to-noise level of the APD. As is well known in the art, gain processes are random in an APD and the shot noise, IS, for an APD is defined as follows:IS=√{square root over (2qIM2F)}  (1)where M is the gain, F is the excess noise factor, q is the charge and I is the gain multiplied current for M=1. The excess noise factor is a function of the ratio, k, of ionization coefficients of holes αh and electrons αe, or k=αh/αe. The excess noise factor quantitatively defines the excess noise for the APD, such that the APD will have excess noise if F is greater than 1 when M is greater than 1. Assuming that another high reverse bias breakdown mechanism, such as dark current tunneling, is absent in an APD, the excess noise factor determines at what gain value the APD noise reaches the system noise and, hence, the minimum noise equivalent power (“NEP”).
The original model for excess noise in early generation APDs (“McIntyre model”) assumed a constant field, a constant k in the carrier multiplication region and a history independent carrier ionization process. See McIntyre, R. J., 13 IEEE Trans. Electron Device at 164 (1966), incorporated by reference herein. Minimum noise is realized when the minority carriers generated in the absorption region of the APD are the dominant multiplying species. According to the McIntyre model, the excess noise factor, Fn, for an nth electron injection APD, which is one of a plurality of such APDs constituting pixels in a detector array, is a function of k and defined as follows:
                              F          n                =                              M            n                    ⁡                      [                          1              -                                                                    (                                          1                      -                      k                                        )                                    ⁡                                      [                                                                  (                                                                              M                            n                                                    -                          1                                                )                                                                    M                        n                                                              ]                                                  2                                      ]                                              (        2        )            FIG. 1 illustrates the behavior of the excess noise factor versus gain as a function of k in an electron injection APD according to the McIntyre model. Referring to FIG. 1, in an electron injection APD (“EAPD”), electron minority carrier generation and multiplication is preferred, in other words, a low k is preferred to achieve the lowest noise. In contrast, in a hole injection APD where the hole minority carrier is the dominant multiplying species, a large k achieves the lowest noise.
In early generations of APDs fabricated in silicon, germanium and like semiconductor materials, it was found that the excess noise increased with the gain in agreement with the McIntyre model, and that the shot noise (see Equation (1)) increased faster than the gain. Once some minimum level of gain was present in such early generation APDs, the excess noise started to degrade the signal-to-noise performance. The gain at which signal-to-noise performance began to degrade in such APDs depended on the pre-amplifier noise or the noise level of the detector system.
In addition, early generations of APDs fabricated in HgCdTe and like semiconductor materials had the characteristic of premature dark current breakdown, in other words, generation of high levels of dark current at high reverse bias (high gain). This characteristic prevented practical operation of such APDs at high gain. The dark currents at increasing reverse bias (higher gains) were so high in such APDs that they masked the excess noise behavior of the APDs. These high dark currents were attributable to defects that were formed in the semiconductor materials of such early generation APDs and were the result of the semiconductor manufacturing techniques and materials existing in the prior art at that time. Early generation HgCdTe and like material APDs, thus, typically were operated only at relatively low gains and, for some time, there was little or no desire in the prior art to further research and develop such APDs for high gain operation.
Over the years, semiconductor fabrication processes and semiconductor materials technology improved. Later generations of APDs contained less defects in the semiconductor material and improved architectures for limiting noise due to premature dark current breakdown effects. It was eventually found that operation of later generation electron injection photodiodes fabricated in HgCdTe in an avalanche mode provided for relatively high gain and low, gain independent excess noise. See Beck 2001. For example, it was found that a later generation EAPD fabricated in HgCdTe has a k approximately equal to zero. The improvement in excess noise performance in such later generation EAPDs was significant in comparison with the excess noise performance of early generation EAPDs made from InGaAs and Si, which had k values equal to 0.45 and 0.02, respectively and, therefore, had the characteristic of increasing excess shot noise with increasing gain.
As known in the art, when the level of reverse bias applied to an APD is increased to obtain higher gains, tunnel breakdown may start to occur, such that the dark current generated in the APD substantially increases. Although dark current in early generation wide bandgap APDs was relatively low so that dark current was not a primary concern in such APDs even at higher gains, it was well known in the prior art that there was a substantial presence of dark current at increasing gain in early generation narrow bandgap APDs, such as HgCdTe APDs. Early generation HgCdTe APDs, thus, had a high gain normalized dark current at high gains. As known in the art, increases in the gain normalized dark current of an APD adversely affect signal-to-noise level performance.
The discovery that later generation HgCdTe APDs had a low, gain independent excess noise characteristic was significant in the detector field, but this discovery did not provide any information regarding the contribution of dark current to the noise in such later generation APDs, especially at high gain applications. Consequently, there was a continuing expectation in the prior art that high levels of dark current would exist at high gains in later generation narrow bandgap APDs, including those narrow bandgap APDs exhibiting gain independent excess noise such as the HgCdTe APDs described in Beck 2001. This expectation in the prior art of high dark current at high gain served as a disincentive for operating later generation narrow bandgaps APDs, such as the later generation HgCdTe APDs described in Beck 2001, at higher gains, despite the finding that such APDs had a gain independent excess noise factor.
Therefore, there exists a need for a method of operating an avalanche photodiode at an avalanche gain that reduces the gain normalized dark current for the photodiode.